High Efficiency Selective Calling Radio System

Abstract

Selective calling radio systems based upon binary pulse coding have not been considered practical because of vulnerability to pulsive urban noises. By adopting a new decoding method which is able to discriminate code length and a suitable integration system, and by increasing the Hamming distance of codes, a new system has succeeded in drastically reducing the probability of WRONG CALLs, and has made it possible to achieve highly efficient selective calling under severe noise situations. Analyses were made on the disturbances by impulse noises to PCM-FM signals, and it was found possible to evaluate the call rate as a function of the number of noise impulses per unit time. Experiments were carried out for a system with 1 048 576(2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sup> ) subscribers. The minimum Hamming distance of codes was selected to be 2 bits, the integration format was 2/5, the code length was 21 bits, and receivers were made pocket-sized. The maximum rate of WRONG CALL is <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3.17 \times 10^{-4}</tex> per second, and CALL capacity is 47 500 subscribers per hour under a noise situation which gives a CALL rate of 50 percent for a one-train code. The field intensity needed for a CALL rate of 90 percent is 35 dB <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</inf> V/m with an ambient noise of 40 dB <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</inf> V/m peak at 150 Mc/s.